The present invention relates to a method for forming an alignment mark required when forming a CMOS transistor on a semiconductor film provided on a support substrate through an insulation film.
There is shown in FIG. 4A to FIG. 6E a conventional method for forming an alignment mark used in forming a CMOS transistor. On a semiconductor substrate 20, an oxide film 21 is formed to 500 angstroms and further a nitride film 5 is formed to 1000 angstroms on the oxide film. Next, a resist 6 is applied and patterned on the nitride film 5 to form an opening 8 for forming a well. At this time, an alignment mark 7 is also formed. The alignment mark 7 is also an opening in the nitride film. Next, ion implantation for formation of a well 9 is conducted with the resist 6 and nitride film 5 serving as a mask, wherein ions are implanted penetrating through the oxide film 21 in the opening 8 formed in the nitride film 5. Further, as shown in FIG. 5A, an oxide film 22 is formed on the ion-implanted semiconductor substrate 20 to 3000 angstroms using the nitride film 5 as a mask. At this time, oxidation is effected in an alignment 7 area. This oxidation process is required to provide on the semiconductor substrate 20 a step for an alignment mark 7 serving as a mark for alignment utilized in a succeeding LOCOS forming process. Next, as shown in FIGS. 5B and 5C, the nitride film 5 is removed to perform well drive-in. Further, the oxide films 22 with the film thickness of 3000 angstroms are removed from the areas of the well 9 and alignment mark 7. This forms a step (recess) 23 for an alignment mark to approximately 1000 angstroms on the semiconductor substrate (FIG. 5D). Next, an oxide film 24 is formed to 350 angstroms on the semiconductor substrate (FIG. 5E), and a nitride film 12 is further formed thereon to 1600 angstroms as shown in FIG. 6A. Next, a resist 13 is applied on the nitride film 13 and then patterned matched to the alignment mark 23 with a recess of approximately 1000 angstroms formed on the semiconductor substrate 20. This patterning is for LOCOS formation. Then, the nitride film 12 is etched to remove the nitride film 12 from areas 14 where a LOCOS 15 is formed. At this time, a LOCOS 15 is also formed in an alignment mark 25 area. Next, as shown in FIG. 6D, oxidation is made to 5500 angstroms in areas 14 for LOCOS formation where nitride film 12 has been removed. This forms oxide films as LOCOS 15 to 5500 angstroms in areas not covered by the nitride film 12. Further, an oxide film is formed to 5500 angstroms for an alignment mark 25. Accordingly, the alignment mark is given a step (convex) of approximately 2000 angstroms. Next, as shown in FIG. 6E, the nitride film 12 is removed and further the oxide film 24 is removed from areas not formed with LOCOS 15, followed by advancing to a gate oxide film forming process. It should be noted that, in alignment processes from now on, patterning will be conducted matched to convex form of approximately 2000 angstroms formed in the LOCOS process.
In the conventional CMOS transistor forming method, there are oxide film forming processes in a total thickness of about 3500 angstroms before performing the well drive-in process. Here, if these oxide film forming processes are implemented by thermal oxidation, oxidation advances also in a thickness direction of the semiconductor substrate. If the conventional CMOS forming method is conducted on an SOI wafer having a semiconductor film thickness of from 200 to 3000 angstroms on an insulation film, the oxide film advances in a depth direction of the semiconductor film during an oxide film forming process. This results in a change of the entire semiconductor film region into an oxide film before the well drive-in process. In order to avoid such a disadvantage, it is a known practice to take a measure of reducing the oxide film thickness during the oxide film forming processes to be conducted after an ion implant process for well formation. In the conventional CMOS transistor forming process, however, steps (recesses) made upon removing, after oxide film formation, an oxide film from areas of wells and alignment marks are utilized as alignment marks in a succeeding process. Consequently, with such a measure of reducing oxide film thickness, a formed alignment mark is small in step (recess) size. Due to this, it is difficult to confirm the alignment marks upon implementing alignment in later process steps.
Meanwhile, although convex-formed alignment marks are also formed in the process of forming LOCOS, the formation of LOCOS advances oxidation also in the depth direction of a semiconductor film. Accordingly, for a wafer with a semiconductor film of 200 to 3000 angstroms, LOCOS will connect an insulation film on a support substrate, making it impossible to increase LOCOS thickness. Accordingly, the alignment marks are smaller in step dimension. Thus, there has been difficulty in implementing patterning with such alignment marks in the process of forming LOCOS and the later steps.
It is therefore an object of the present invention to form an alignment mark that is to be confirmed in later processes.
In order to solve the above-mentioned problems, the present invention comprises: a step of forming an oxide film on a semiconductor film of 200 to 3000 angstroms provided on a support substrate through an insulation film, a step of forming a nitride film on the oxide film, a step of etching the nitride film, a step of performing ion implantation through an opening in the etched nitride film to form a well, a step of etching the nitride film, the oxide film, the semiconductor film and the insulation film on the support substrate to reach a part of the support substrate to form an alignment mark, a step of removing the nitride film and the oxide film and forming a new oxide film and a new nitride film, a step of etching the new nitride film, a step of oxidizing where the new nitride film has been etched in an opening area to form a LOCOS, and a step of removing the new nitride film. Due to this, an alignment mark is formed also on the SOI wafer support substrate. This eliminates the disadvantage that the alignment mark is small in step size and hence impossible to confirm as having been encountered so far in the method of forming a recess-formed alignment mark in an oxidation film process before well drive-in.
Meanwhile, a convex-formed alignment mark is formed in a process of forming LOCOS. For a wafer with a semiconductor film of 200 to 3000 angstroms, however, the formation of LOCOS advances oxidation also in a depth direction of the semiconductor film, resulting in connection with an insulation film on the support substrate. Consequently, a disadvantage is eliminated that the thickness of LOCOS cannot be increased and a mark is difficult to confirm with an alignment due to a small step in the alignment mark. The present invention has an effect to eliminate this disadvantage.